Increasing demands in the aerospace industry are creating need for lightweight structural materials having increased strength-to-density and increased stiffness-to-density at high temperatures. As high temperature applications have exceeded 1200.degree. C., increased attention has been directed to ceramics such as alumina and silicon carbide. However, the design problems associated with the brittle nature of ceramic materials and the difficulty of fabrication have presented severe obstacles.
Fiber-reinforced composite ceramic articles or CMC's (ceramic matrix composites) are receiving increasing interest in aerospace applications that require properties such as high chemical, wear and corrosion resistance and good structural integrity at high temperatures. Such reinforced ceramics are presently being considered as suitable structural materials for the fabrication of heat exchangers, turbocharger rotors, cylinders, bearings, and other components of heat engines. The incorporation of fiber reinforced ceramic matrix composites will permit heat engines to run more efficiently at higher temperature than heretofore possible with similar components of metal.
Reinforcing ceramic oxides such as alumina (Al.sub.2 O.sub.3) and alumina phosphate with whiskers or fibers has produced some particularly useful fiber-reinforced composite ceramic materials (see for example U.S. Pat. Nos. 4,158,687, 4,358,500, 4,563,219 and 4,652,413). The fibers function in the composite to substantially increase the fracture toughness of the matrix and thereby inhibit deleterious crack formation and crack growth due to material fatigue.
Silicon carbide reinforced ceramic composite materials have been shown to significantly increase the strength and fracture toughness over conventional ceramics. However, their strength, structural reliability and impact resistance have not met the demands of the high temperature applications (in excess of 1500.degree. F.) for which they are being designed.
In addition to thermal performance problems, there are still problems that exist with the fabrication processes that are presently being used to make fiber reinforced CMCs. Many of the current fabrication techniques are limited to the formation of fiber reinforced ceramic articles which possess relatively simple shapes and shallow thicknesses. For example, the hot pressing of ceramic particulate mixtures in die sets inhibits the fabrication of articles with relatively complex shapes such as turbocharger rotors, cylinders, bearings, and the like. In addition, the hot pressed ceramic reinforced composites require extensive machining and other finishing processes which considerably increase the expense of fabricating structural components of complex shapes.
Another fabrication technique that limits the shapes and thicknesses of fiber reinforced CMCs are those that utilize a binder solution containing a fugitive organic binder. The fugitive organic binder is used in the slurry matrix to improve the adhesion of the matrix to the fibers before sintering. The organic fugitive binders evolve gases that must be completely removed prior to sintering so that voids are not created in the sintered piece. As the cross sectional thickness of the piece increases, the time that the green ceramic article must remain in an oven at a low burn off temperature increases. In addition, as the size and cross sectional thickness of the article increases, the likelihood that the evolved gases will be trapped in the interior of the piece also increases. Therefore, fabrication of pieces by techniques employing fugitive organic binders must employ an additional production step to insure the binders have burned off prior to sintering. This extra step may be quite lengthy and adds both labor costs and energy costs to the process.
Another disadvantage associated with above composite article manufacturing technique, which employs a fugitive organic binder, is that the organic fugitive binders can be a source of unwanted impurities in the resultant composite. The unwanted impurities often result in a lowering of the high temperature properties of the composite article.
Yet another disadvantage of current fabrication techniques is that the matrix does not uniformly surround the fibers. The nonuniformity of the green body results in a nonuniform sintered composite article which may contain voids. This problem can be alleviated to a certain extent by the use of binder solutions which carry ceramic particles further into the fabric (fibers) and into the interior of the article. However, often the fabric acts as a filter which removes ceramic particles from the binder solution. This results in the matrix having a composition which changes as one moves into the interior of the article. This problem of nonuniformity is further aggravated when forming complex and/or three-dimensional composite structures. The problems of nonuniformity can be overcome, however often the labor costs needed to insure compositional uniformity may make the cost of the final product uncompetitive.
Another problem associated with the use of the prior art fabrication techniques is that they involve the mixing of reactive chemicals. Thus for example, in U.S. Pat. No. 3,730,744 the ceramic particles are bonded together by an aluminum dihydrogen orthophosphate bonding solution which is prepared by the reaction of reactive aluminum orthophosphate (AlPO.sub.4), aluminum hydroxide or colloidal alumina with hot, concentrated phosphoric acid. In U.S. Pat. No. 4,440,865, the aluminum silicate which is chemically bound by an aluminum phosphate composition formed from liquid aluminum phosphate and phosphoric acid. In U.S. Pat. No. 4,563,219, the inorganic binder solution employed in providing refractory coatings on fabric substrates is prepared from colloidal silica, monoaluminum phosphate (MAP) and aluminum chlorohydrate (ACH) and a catalyst of an alkyl tin halide. U.S. Pat. No. 4,358,500 discloses that the colloidal silica and water in the binder solution act as a moderator to delay the rapid exothermal chemical reaction that would normally occur when MAP is mixed with ACH.
It would be advantageous, therefore, to provide a method of fabricating fiber-reinforced ceramic matrix composites which exhibit a high flexure strength at temperatures in excess of 1500.degree. F. In addition, it would be advantageous to provide a process for economically forming fiber-reinforced ceramic matrix composite articles into relatively complex geometries and sizes.
The principal object of the present invention is to provide a composition for producing a fiber-reinforced aluminum phosphate matrix composite which has a high flexure strength at temperatures in excess of approximately 1500.degree. F.
Another object of the present invention is to provide a method of producing a fiber-reinforced aluminum phosphate matrix composite, which has a high flexure strength at temperatures in excess of approximately 1500.degree. F., that does not involve processing at high temperatures and pressures. These conditions lead to mechanical and chemical damage of the fiber and unfavorable reactions between the fiber and matrix material.
Still another object of the present invention is to provide a low-cost process for producing a fiber-reinforced aluminum phosphate matrix composite that does not require mixing reactive chemicals such as aluminum hydroxide and hot, concentrated phosphoric acid.
A further object of the present invention is to provide a low-cost process for producing a fiber-reinforced aluminum phosphate matrix composite that does not require the addition of colloidal silica in the binder solution to act as a moderator to delay the rapid exothermal chemical reaction that might otherwise occur when using monoaluminum phosphate (MAP) and a reactive chemical like ACH.
Yet another object of the present invention is to provide a low-cost process for producing a fiber-reinforced aluminum phosphate matrix composite that does not require the addition of contaminating fugitive organic binders, which may produce impurities, to improve the adhesion of the matrix to the fibers before sintering.
Another object of the present invention is to provide a process for economically forming fiber-reinforced ceramic matrix composite articles into relatively complex geometries and sizes.
Another object of the present invention is to provide a method of manufacturing fiber-reinforced ceramic matrix composite articles which is less complex, less labor- and energy-intensive and allows for easier production of complex composite articles.
These and other objects and advantages will be more fully understood and appreciated with reference to the following description.